Myostatin (Mst) is a negative regulatory protein of skeletal muscle mass during embryological development. Spontaneous or induced inactivating mutations of Mst are associated with considerable increase of skeletal muscle, suggesting that myostatin is an important genetic determinant of muscle mass. Elevated levels of Mst are associated with skeletal muscle loss in adult animals and humans, but a clear cause/effect relationship has not been established, and the role of Mst protein in regulating muscle mass in adult animals remains poorly understood. In this proposal we hypothesize that a transgenic mouse able to hyperexpress myostatin in the skeletal muscle by pharmacological modulation, will lose muscle mass and impair its function in response to the activation of this process, through a reduction in the number and size of muscle fibers, and that these effects are age and gender dependent. This would demonstrate conclusively that Mst is a key negative regulator of muscle mass and function in postnatal life. The following aims are proposed: 1) We will generate a conditional Mst hyperexpression transgenic (CMHT) mouse, where Mst expression is activated at the desired periods in adulthood by doxycycline (Dox). We will use the gene expression system, Tet-On, based on a regulatory construct driven by the mouse muscle creatine kinase (MCK) promoter, and a response construct of the mouse Mst cDNA, or alternatively a single construct with sequences in tandem. Constructs will be tested in vitro in HEK 293 Tet-On approved transformed primary embryonal kidney cell and C2C12 myoblast, myotube cells and used to originate the respective transgenic animals. 2) We will turn recombinant Mst expression on in male and female mice at the ages of 7 weeks, 6 and 18 months, for different periods, and measure muscle and fat mass, fiber size and composition, muscle strength and endurance, and energy expenditure, in correlation with myostatin expression, determined by real time RT/PCR, northern and western blot. 3) We will determine whether turning off recombinant myostatin expression will reverse the effects in an age- and gender-dependent process that can be modulated by exercise training. This study may conclusively determine that hyperexpression of myostatin causes muscle wasting in the adult animal and may open the way to prevent or reverse aging-related sarcopenia via the inhibition of Mst expression and/or activity.